Although patients with breast carcinoma mount a T cell-mediated immune response to their disease it usually progresses. Among the many escape mechanisms identified, defects in HLA class I/tumor antigen(TA) peptide complex expression by tumor cells have attracted much attention since these complexes are involved in the generation of HLA class I restricted cytolytic T cell (CTL) immunity. Previous studies have shown that HLA class I antigens are frequently down-regulated in breast cancer cells. However, it remains unknown as to how this down-modulation occurs and whether HLA-class I down-modulation correlates with the level of HLA class I/TA peptide complex expression. The lack of this information reflects the limited availability of probes to evaluate expression of HLA class I/TA peptide complexes on tumor cells, including breast carcinoma cells. In recent years, we have acquired new antibody probes that could potentially answer many questions related to loss of HLA-class I expression on breast cancer cells. These probes include single chain Fv fragments that are able to recognize HLA-class I/TA peptide complexes as well as numerous monoclonal antibodies that recognize multiple components of the antigen processing and presentation machinery. We will use these probes to define the defects that lead to the loss of HLA-class I/TA peptide complex expression, how these defects that lead to tumor immune evasion, and the relationship of this evasion to the clinical course of the disease. The model antigen we will use to examine HLA-class I expression is HER-2/neu, an antigenic protein overexpressed on nearly 20-30% of breast cancers. We will then examine in xenograft models whether upregulation of the complexes in vivo can improve sensitivity to T cell killing. The specific aims are: To examine whether the affinity, avidity, and stability of scFVs, that recognize HLA-A*0201:HER2/neupeptide complexes on breast cancer cells, can be improved with molecular modeling techniques (2) To examine whether defects in the APM downregulate HLA-A*0201-HER2/neu peptide complexes on breast carcinoma cells, (3) To examine whether HLA-A* 0201-HER2/neu peptide complex downregulation reduces breast cancer cell recognition and cytotoxicity by HLAA* 0201-HER2/neu peptide-specific CTL, (4) To examine whether HLA-A*0201-HER2/neu peptide complex downregulation in breast cancer lesions has a negative impact on the clinical course of the disease, and (5) To examine whether exogenous agents can restore normal expression of APM components and HLA-A*0201-HER2/neu peptide complexes in vivo. We expect that the results from this study will lead to an improved understanding of the impact of HLA-class I loss on tumor immune evasion. The results will likely lead to the identification of specific molecule associated with antigen presentation that could be either a novel therapeutic target or biomarker of disease outcome.